Obstructive Sleep Apnea (OSA) is a high-prevalence disease, especially in the male adult population. OSA is very effectively treated by the application of Positive Airway Pressure (PAP). This involves the patient wearing a mask at night, which delivers pressurized air to the patient during sleep.
For an optimal pressure titration an Automatic PAP (“APAP”) system has to distinguish between central and obstructive events. A central sleep apnoea (CSA) event arises when the patient makes no effort to breath whereas an obstructive event arises when there is a physical blockage of the upper airway. Both central and obstructive events occur during sleep repeatedly, and an event may last for at least ten seconds up to a minute or slightly more. Patients may suffer from essentially OSA, essentially CSA or a combination of both, the latter being referred to as Mixed Sleep Apnea.
When a known PAP system detects a complete cessation of airflow it sends a pressure pulse to verify if the drop in airflow is caused by an obstructive or a central apnoea event. If the pressure pulse (typical duration of 2 s, and typical pressure increase of 2 mbar) leads to an increase of airflow the apnoea will be a clear airway apnoea (CA) such as a central apnoea event. If the pressure pulse does not increase the airflow, the system knows the apnoea is an obstructive apnoea (OA).
However in case of hypopnea events (shallow breathing events rather than breathing interruptions), there is a reduction of airflow typically of less than 40%. The PAP system cannot distinguish between central and obstructive hypopnea events. In both cases a pressure pulse leads to an increase of airflow because the airway is still at least partially open. In the case of central hypopnea the reduction of flow is caused by a reduction of the neuromuscular respiration drive, whereas in case of obstructive hypopnea the reduction is caused by a narrowing of the airway, which leads to an increase of the upper airway resistance.
In the case of an obstructive hypopnea, an increase of the CPAP pressure is beneficial to achieve airway patency. In the case of a central hypopnea a pressure increase will not increase the airflow, it might be even contra indicative; an unneeded pressure increase may lead to discomfort, lowering compliance of the patient to use the system.
Existing PAP systems are not able to measure either the respiration drive nor the airway resistance. The resistance of the whole respiratory system (covering the upper airway and the lung) can however be measured by using Forced Oscillation Technology (FOT). The FOT technique modulates the pressure in the airway by a low frequency sine wave excitation (typically 1 mbar). The used frequencies are less than 20 Hz. Such an analysis in the frequency domain can only determine the overall resistance of the lung, the larynx and the upper airway but does not provide the spatial information in the time domain to localize the segment in the upper airway, which causes a change of the resistance.
Thus, there is a first problem that current PAP systems cannot determine whether a hypopnea is caused by upper airway narrowing or by a reduction of respiratory drive.
Some patients adapt poorly to PAP treatment because of its obtrusive properties. As a result, more and more patients seek an alternative treatment, and this is particularly the case for patients suffering from mild to moderate OSA.
The pathophysiology of OSA is complex as it often results from an interplay of anatomical and neuromuscular dysfunctions. The power of PAP therapy is that it treats all collapsible levels of the upper airway and therefore works for every OSA patient, regardless of the pathophysiological causes. Although many treatment alternatives have higher patient acceptance, they only treat a specific level of the upper airway. This makes the applicability of these alternatives restricted to OSA sub-populations.
The inability of PAP treatment alternatives to treat all levels of the upper airway at the same time has as a consequence that patient selection becomes key for these alternatives to ensure optimal clinical outcomes. This requires a deeper study of OSA pathogenesis in those patients eligible for PAP alternatives.
Acoustical techniques to evaluate the upper airway are known in the art and the pharyngometer of Eccovision, and the Rhinometry system of Hood Laboratories are examples.
U.S. Pat. No. 8,424,527 discloses a system in which an acoustic transducer is integrated in a PAP mask to study airway narrowing under applied airway pressure. A single sensor functions as a microphone and a sound source. US 2013/0046181 discloses a collar a patient wears around the neck, which uses acoustic pulses to image airway narrowing. These examples demonstrate the feasibility of acoustics to resolve upper airway properties. These examples have in common that they analyze the scattered sound of active sound sources provided by speakers/transducers.
There is thus a second problem that airway analysis and diagnosis systems, such as used to enable suitable non-PAP treatments to be selected, can be obtrusive in their measurement techniques, mainly because they are not suitable during normal sleep. For example it is not desirable to create sound which disturbs the user, and the system needs to be minimally obtrusive to the user.